A mass storage device, such as a hard disk drive, utilizes a spindle motor to rotate platters that store information. The spindle motor generally rotates the platters at a constant angular speed while electromagnetic heads read from or write to circular tracks on the platters. The spindle motor is often implemented as a three phase motor.
A typical, three-phase motor includes a magnetic rotor and three electric coils. The three electric coils are related to the three phases of the motor. A separate current, called a phase current, flows through each of the three electric coils of the motor. The rotor rotates in response to an electrical field created by the phase currents.
The phase currents flowing through the three electric coils generally interact with the magnetic elements in the motor to produce acoustic noise. In some traditional drives, each of the motor's three electric coils cycles through three states: each electric coil can be held at ground, driven to some positive voltage, or floated. As a result, the phase currents flowing through the electric coils have very abrupt transitions. Because torque is proportional to current, the abrupt changes in the phase currents cause the torque also to change abruptly. If the harmonics in the torque waveform excite mechanical resonances, the motor structure may vibrate and generate audible noise.